The metabolism of feed by ruminant animals such as cattle, sheep and goats having a developed rumen function has been a target of intensive investigation in recent years. It has been discovered that by improving the efficiency of rumen fermentation, a corresponding increase in the rate of growth and/or an increase in the efficiency of feed utilization by the animals will occur.
The overall efficiency of rumen fermentation is a function of the symbiotic activities of the microbial species inhabiting the rumen. These microbial species are responsible for the transformation of carbohydrates as well as protein and nonprotein nitrogenous substrates into forms such as microbial cell protein and volatile fatty acids suitable for biochemical utilization by the ruminant animal. The exact composition of the end products is a consequence of competition between the microbial species for the substrates or nutrients. The efficiency of the rumen fermentation is an important factor in determining the degree of feed utilization efficiency and/or the rate of growth of the animal.
For example, during nitrogen metabolism, a portion of the feed protein (depending on the type of protein) is hydrolyzed by microbial enzymes in the rumen to ammonia and isoacids which are subsequently fixed into microbial protein. Other proteins from feed sources are metabolized to peptides and free amino acids. The peptides may subsequently be transformed into free amino acids by certain ruminal bacteria (for example, Bacteroides ruminicola) leaving a pool of free amino acids which may then be assimilated into microbial protein or catabolized to produce energy for microbial growth. The free amino acids may also be assimilated directly by the animal and used for protein synthesis or catabolized as a source of energy.
Carbohydrate metabolism provides energy for the growth of rumen microbes primarily through the fermentation of cellulose and starch. The insoluble polymers are converted to oligosaccharides and soluble sugars by extracellular enzymes from the rumen microorganisms. The resulting sugars are then fermented to one of various forms of volatile fatty acids, carbon dioxide and hydrogen. As used herein, the volatile fatty acids - acetic acid, propionic acid and butyric acid - are also referred to as acetate, propionate and butyrate, respectively. Volatile fatty acids are utilized by the animal as primary carbon and energy sources with varying degrees of efficiency. High levels of propionic acid are desirable because propionic acid is a primary metabolic precursor for gluconeogenesis in the animal. The fermentation of 6-carbon sugars to acetic acid is relatively inefficient since in this process, carbon is lost via eructation in the form of carbon dioxide or methane. On the other hand, the production of propionic acid does not result in a loss of carbon.
Rumen metabolism studies have shown that some of the rumen microbes such as various Ruminococcus and Butyrivibrio species ferment the monosaccharides of complex carbohydrates to formic, acetic, butyric and succinic acids, along with carbon dioxide and hydrogen. The carbon dioxide and hydrogen produced during fermentation are used in the formation of methane through the activity of methanogenic bacteria. Rumen microorganisms such as various Bacteroides species ferment carbohydrates predominantly to succinic acid which is converted to the desirable propionic acid by various Selenomonas species or other microorganisms.
It becomes possible then to improve feed utilization efficiency and/or the rate of growth of ruminant animals by selectively inhibiting or stimulating the growth of ruminal microflora involved in rumen fermentation. For instance, feed utilization efficiency and/or rate of growth can be improved by increasing the molar proportion of propionic acid to acetic acid or by increasing total volatile fatty acid concentration (i.e. the sum of acetic, propionic and butyric acids) in the rumen. For example, an increase in the molar portion of propionic acid can be accomplished by selectively inhibiting the growth of various species of Ruminococcus and Butyrivibrio and by stimulating the growth of various species of Bacteroides and Selenomonas. Likewise, it is also known that inhibiting methanogenesis in the rumen results in an apparent decrease in gaseous loss of methane via eructation and a shift toward producing more desirable fatty acids for growth, especially propionic and butyric acids. See U.S. Pat. Nos. 3,745,221; 3,615,649; and 3,862,333. Higher levels of the metabolic end products of feed protein degradation (such as ammonia and isoacids) can lead to such beneficial effects as a stimulation of microbial protein synthesis. In addition, a partial inhibition of the deamination activity of the rumen microflora makes more of the amino acids available for the nutrition of the animal.